The invention relates to processes making use of a reagent gas containing one or more gaseous hydrocarbons, in particular processes for cementation of parts, for forming coatings of pyrolytic carbon on substrates by chemical vapor deposition (CVD), or for densifying porous substrates with a pyrolytic carbon matrix formed by chemical vapor infiltration (CVI).
One particular, but non-exclusive, field of application of the invention lies in making composite material parts comprising a fiber reinforcing substrate or “preform” densified by a matrix of pyrolytic carbon, and in particular carbon/carbon (C/C) composite material parts.
Substrates for densification are placed in an oven into which a reagent gas containing one or more precursors is introduced at low pressure. The precursor is constituted by one or more gaseous hydrocarbons, typically methane, propane, or a mixture thereof. The operating parameters of the oven are adjusted so as to produce the pyrolytic carbon matrix by decomposing (cracking) the precursor gas in contact with the substrate. An effluent gas containing reaction by-products is extracted from the oven by pumping.
Usually, the operating parameters of the oven, in particular the temperature of the oven, the pressure inside the oven, the flow rate of reagent gas through the oven, and the composition of the reagent gas, are constant throughout the densification process. Unfortunately, infiltration conditions change as the process advances because the initial pores within the substrate become progressively filled in. The selected parameters thus result from seeking the best compromise between the optimum that is suitable for the beginning of densification and the optimum that is suitable at the end of densification, but with a risk of a modification to the microstructure of the deposited matrix material because of the changing pore size of the substrates, i.e. the changing geometrical characteristics of the pores. Adapting the operating parameters of the oven to progress in the densification process could enable the process to be optimized overall, thereby reducing the time needed to obtain a desired level of densification and ensuring that the matrix material is formed with the desired microstructure.
Thus, in document WO 96/31447, the Applicant has proposed varying the operating parameters of the oven so as to optimize the densification process while controlling the microstructure of the matrix material. Nevertheless, that variation is undertaken in application of a pre-established model, and does not take account of the real progress of the process.
Proposals are also made in document U.S. Pat. No. 5,348,774 to measure variation in substrate weight on a continuous basis so as to monitor progress in the densification process. As a function of the measured variation, it is possible to act on various parameters, in particular the power supplied to an induction coil which serves to heat the oven, by coupling with a susceptor defining the side wall of the oven. Monitoring variation in the weight of the substrates also makes it possible to detect when the densification process has come to an end. However, this requires the oven to be specially arranged to make it possible to measure substrate weight on a continuous basis, even at the high temperatures that exist in the oven. Such arrangements can also be penalizing on the substrate-loading capacity in the inside working volume of the oven.